1. Field of the Invention
The subject invention relates to fuel pumps for gas turbine engines, and more particularly, to vane pumps wherein pressurized fluid is supplied to the undervane portion of the vane elements to balance forces imparted thereon.
2. Background of the Related Art
Fixed displacement and variable displacement pumps are used as main fuel pumps in the aviation gas turbine industry. An example of a fixed displacement vane pump is disclosed in U.S. Pat. No. 4,354,809 to Sundberg and a variable displacement vane pump is disclosed in U.S. Pat. No. 5,545,014 to Sundberg et al. The disclosures provided in these patents are herein incorporated by reference to the extent they do not conflict with the present disclosure.
Vane pumps traditionally include a housing, a cam member, a rotor and journal bearings. The housing defines an interior chamber, a fluid inlet and a fluid outlet and the cam member is disposed within the interior chamber of the housing and has a central bore which defines the circumferential boundary of the internal pumping chamber. Mounted for rotational movement within the central bore of the cam member, is a rotor supported by axially opposed journal bearings. Typically, the rotor element has circumferentially spaced apart slots machined therein which support corresponding radially-movable vane elements. The vane elements have a radially outer tip portion which slidably contacts the circumferential portion of the internal pumping chamber and a radially inner undervane portion.
In a single rotation, the vanes of the rotor element of the pump traverse at least four distinct arcuate regions which make up the 360 degree revolution. The first region is the inlet arc segment in which fluid is received into the pumping chamber and over this region the bucket volume increases. The second region is the discharge arc segment in which pressurized fluid is discharged from the pumping chamber and throughout this region, the bucket volume decrease. Lastly, seal arc segments separate the inlet and discharge arc segments and represent the arc segment through which the bucket volume remains substantially constant.
In operation, fluid at a first pressure is fed into the pumping chamber through the housing inlet, and into the space defined between adjacent vane elements, known as the bucket. In positive displacement vane pumps, as the vane elements rotate within the pumping chamber from the inlet region to the outlet region, the configuration of the cam member causes the vanes to retract within the corresponding slots. This causes the volume defined by the bucket to decrease. Since the amount of fluid received into an inlet bucket is greater than that contained within the corresponding discharge bucket, a fluid volume equivalent in size to the volumetric difference is discharged or displaced through the outlet port at a pressure equal to the downstream pressure which must be overcome.
Typically, pumping pressures and velocities are so high within a pump housing that the use of heavy, high wear resistant materials such as tungsten carbide for the vanes and cam member becomes necessary to handle the wear which is caused by these high levels of pressure and velocity.
During this rotation, a radially outward centrifugal force is exerted on the vane elements. At the same time, pressurized fluid within adjacent buckets acts to force the vane elements radially inward. Often, the forces applied to the vanes are not balanced and therefore, the vane tip is either subjected to excessive wear or fluid leaks from within the bucket. This reduces pumping efficiency.
The ideal operating condition for a pump is when the pressure applied to each vane element is balanced and each vane element “floats” within a corresponding slot in the rotor. This condition results in minimum wear to the vane tips and minimum pressure losses due to the lack of contact between the vane tips and the cam member.
Prior attempts at correcting the unbalanced vane condition have included applying pressure to the undervane portion of the vane. In general, the typical vane pump does not incorporate an undervane pumping feature. Those that do, typically supply pressure from within the buckets in the inlet region to the undervane portion of vanes within the inlet arc. Similarly, the undervane portion of the vanes within the discharge arc are supplied with pressure from the buckets located in the discharge arc. This feature creates a balanced condition within the inlet and discharge arc regions, but does not correct the unbalanced condition in the seal arc regions.
When the vanes are in the first seal arc region, which is located after the inlet arc region and before the discharge arc region, the leading face of the vane is subjected to pressure from the discharge side of the pumping chamber and the trailing face is subjected to pressure from the inlet side of the pumping chamber. Therefore supplying pressure from either the inlet or discharge arc regions will not balance the forces. In fact, an interim pressure equal to half the discharge pressure plus half the inlet pressure is required to balance the forces imparted on the vanes traversing the seal arc regions.
Examples of vane pumps having pressure-balanced vanes adapted to provide undervane pumping are disclosed in U.S. Pat. Nos. 4,354,809 and 5,545,014. The '809 patent discloses a vane pump incorporating undervane pumping wherein the vanes are hydraulically balanced in not only the inlet and discharge areas but also in the seal arcs. More specifically, the '809 patent discloses a fixed displacement vane pump which utilizes a series of ports machined in the rotor to supply the pressure to the undervane region. Two ports are provided in the rotor on the leading side of the blade and two ports are provided in the rotor on the trailing side of the blade. All of the ports fluidly communicate with the undervane portion of their associated vane element. Although, this configuration provides a balanced condition, ports having a complex configuration must be machined in the rotor at great expense. Also, in pumps which have a seal arc region with an arc length greater than the arc length between the leading and trailing ports, the pressure supplied to the undervane portion is not a mixture of the pressure from the inlet and discharge arc regions, but rather a mixture of the pressure from the seal arc region and either the discharge or inlet arc regions.
U.S. Pat. No. 5,545,014 to Sundberg et al. teaches a durable, single action, variable displacement vane pump capable of undervane pumping, components thereof and a pressure balancing method which is herein incorporated by reference. The '014 patent discloses the use of a servo-piston to supply half discharge pressure to the undervane portion of the vane elements when the vanes are positioned in the seal arc region.
In view of the foregoing, a need exists for an improved vane pump which cost effectively balances that forces exerted on each vane element in the inlet arc region, the discharge arc region and the seal arc regions.